Streams integrate fluxes of water, solutes, and sediment from their catchments, and thus are mirrors of the Streams integrate fluxes of water, solutes, and sediment from their catchments, and thus are mirrors of the surrounding landscape. Conversely, fluxes of water, solutes, and sediments shape the hydrological, chemical, and geomorphological evolution of landscapes, and thus in turn, their drainage systems. In this talk I will summarize several lines of evidence illustrating the coupling between landscapes and the streams that drain them.
In this talk I will summarize several lines of evidence illustrating the coupling between landscapes and the streams that drain them:
1) Groundwater levels and stream flows exhibit diurnal cycles in response to snowmelt in springtime and transpiration during the growing season. These cycles vividly illustrate how aquifers and streams mirror ecological processes in their surrounding landscapes. 2) Active drainage networks dynamically extend and retract, and connect and disconnect, both seasonally and in response to individual rainfall events, dynamically mapping out variations in subsurface transmissivity and in the balance between precipitation and transpiration. 3) Branching angles of valley networks are systematically wider in humid regions than arid ones, consistent with simple models of drainage network evolution under the influence of diffusive subsurface seepage and overland flow erosion. The correlation of mean junction angles with aridity is stronger than with topographic gradient, downstream concavity, or other geometric factors that have been proposed as controls of junction angles. Thus, it may be possible to identify channelization processes from valley network geometry in relict landscapes, such as those on Mars. 4) Stable isotope cycles in rivers indicate that steeper landscapes tend to have have less – not more – young streamflow than flatter landscapes. This superficially counterintuitive result may reflect how steep topographic gradients create rock stresses that fracture bedrock, enhance permeability, and promote deep infiltration. 5) Concentration-discharge relationships in streams are often much flatter than simple dilution models would predict, suggesting that catchments behave like chemostats, with rates of solute production that are nearly proportional to water fluxes, on both event and inter-annual time scales. However, sites with higher average precipitation and discharge have markedly lower mean solute concentrations, suggesting strong dilution of stream chemistry under long-term leaching of the critical zone. The picture that emerges is one in which, on event and inter-annual time scales, stream solute concentrations are chemostatically buffered by groundwater storage and fast chemical reactions, but on much longer time scales, the catchment's chemostatic "set point" is altered by climatically driven critical zone evolution.
Examples such as these will be presented to illustrate how streams can be used as windows into landscape processes, and to outline scientific challenges at the interface between hydrology and geomorphology.